A magnetic recording medium commonly used in audio equipment, video deck and computer applications comprises a non-magnetic support having provided thereon a magnetic layer comprising a ferromagnetic powder dispersed in a binder.
In recent years, there has been a growing demand for high density recording. The demand has been met by decreasing the particle size of ferromagnetic powder or improving the dispersibility of the ferromagnetic powder, the surface properties of the magnetic layer or the ferromagnetic powder packing density.
In particular, an improvement in the dispersibility of ferromagnetic powder has been accomplished by using a dispersant during the preparation of a magnetic coating. In recent years, it has been known that the dispersibility of ferromagnetic powder is improved by using a polyurethane resin containing a metallic sulfonate and vinyl chloride resin as a binder, i.e., by incorporating a polar group into the binder as disclosed in JP-A-61-123017 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). It is also known that the surface properties of the magnetic layer are improved by increasing the temperature or pressure at which calendering is effected, or by grinding the surface of the magnetic layer by means of a grinder as disclosed in JP-A-63-98834. Furthermore, in order to provide a magnetic recording medium which results in reduced clogging of magnetic head and less drop out, it has been proposed to polish the surface of the magnetic layer with a polishing tape as disclosed in JP-A-63-259830. These approaches effectively reduce the number of grannular components such as the ferromagnetic powder, which are readily separated from the surface of the magnetic layer, to thereby remarkably reduce drop out and clogging of the magnetic head. However, when the crystalline size of the ferromagnetic powder particles is less than 250 .ANG., sufficient effects are not obtained by simply employing the above described approaches.
In particular, such a magnetic recording medium which is adapted to providing excellent electromagnetic properties for high density recording generally tends to be disadvantageous with respect to deterioration in running performance and durability. Particularly, clogging of the video head (stain on video head which causes a reduction in output) worsens remarkably when a high density recording magnetic recording medium is used.
Thus, even with the use of the above described approaches, magnetic recording medium products which meet both the requirement for reduced clogging of the magnetic head and excellent running performance have not hitherto been obtained.
In order to improve durability, it has been proposed to use a polyurethane resin containing plurality of hydroxyl groups in combination with a vinyl chloride resin to increase the crosslinking density as disclosed in JP-A-55-139634 and 56-101643. However, dispersibility of the ferromagnetic powder is inadequate.
JP-B-63-58869 and 63-55549 (the term "JP-B" as used herein means an "examined Japanese patent publication") propose the use of a polyurethane resin containing plurality of hydroxyl groups in combination with polar groups.
The present inventors have conducted extensive studies to improve both the dispersibility of the ferromagnetic powder and the smoothness of magnetic layer and to reduce clogging of the video head. As a result, the present inventors have discovered that the ultramicrohardness of the surface of the magnetic layer has a pronounced effect on the surface properties and durability of the surface of the magnetic layer, to thereby reduce clogging of the video head. In particular, it was found that the above described disadvantages are remarkably eliminated by properly selecting the crystalline size of the ferromagnetic powder, the curing conditions of the binder (e.g., vinyl chloride resin and polyurethane resin), and the surface properties of the magnetic layer, such that the ultramicrohardness of the magnetic layer is increased.